Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (“CMOS”) image sensors (“CIS”), has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
Typically, image sensors include an array of pixels arranged in rows and columns. Each pixel may include a photodiode and several transistors to control the accumulation, transfer and resetting of charge accumulated in the photodiode. For example, an image sensor typically includes at least a source follower transistor and a row select transistor for coupling the source follower transistor to a column output line. The pixel also typically has a charge storage node, such as, a floating diffusion node which is, in turn, connected to the gate of the source follower transistor. Charge generated by the photosensor is stored at the storage node. In some arrangements, the image sensor may also include a transistor for transferring charge from the photosensor to the storage node. The image sensor also typically includes a transistor to reset the storage node before it receives photo-generated charges.
For some conventional image sensors, the transistors included in the pixel are driven with a fixed voltage source. However, as pixel sizes continue to shrink and control methods become more complex, the chip area for transferring different drive signals is becoming more limited. Also, driving a transistor in a pixel with a single fixed voltage source may affect performance of the pixel by, for example, introducing unwanted noise.